The Li-ion battery is the premiere high-energy rechargeable energy storage technology of the present day. Unfortunately, its high performance still falls short of energy density goals in applications ranging from telecommunications to biomedical. Although a number of factors within the battery cell contribute to this performance parameter, the most crucial ones relate to how much energy can be stored in the electrode materials of the device.
During the course of development of rechargeable electrochemical cells, such as lithium and lithium-ion battery cells and the like, numerous materials capable of reversibly accommodating lithium ions have been investigated. Among these, occlusion and intercalation materials, such a carbonaceous and graphitic compounds, and transitions metal oxide spinels, have proved to be particularly well-suited to such applications. However, even while performing reasonably well in such recycling electrical storage systems of significant capacity, many of these materials exhibit detrimental properties, such as marginal environmental compatibility and safety, which detract from the ultimate acceptability of the rechargeable cell devices. In addition, some of the more promising materials are available only at costs that limit widespread use.
Materials of choice in the fabrication of rechargeable battery cells, particularly highly desirable and broadly implemented Li-ion cells, have for some considerable time centered upon graphitic negative electrode compositions which provide respectable capacity levels in the range of 300 mAh/g. Complementary positive electrode materials in present cells comprise the less effective layered intercalation compounds, such as LiCoO2 which generally provides capacities in the range of 150 mAh/g. Alternative intercalation materials, such as LiNiO2 and LiMn2O4, have more recently gained favor in the industry, since, although exhibiting no greater specific capacity, these compounds are available at lower cost and, further, provide a greater margin of environmental acceptability.
Due to the increasing demand for ever more compact electrical energy storage and delivery systems for all manner of advancing technologies, from biomedical to telecommunications, the search continues for battery cell materials capable, on the one hand, of providing greater specific capacity over wider ranges of cycling speeds, voltages, and operating temperatures while, on the other hand, presenting fewer environmental hazards and greater availability at lower processing and fabrication costs. Searches for more effective positive electrode materials in particular have become far-reaching with attention turning more frequently to the abundant lower toxicity transition metal compounds, which are typically accessible at economical costs.
In this latter respect compounds of iron, e.g., iron oxides, attracted some attention. However, although exhibiting electrochemical activity, iron oxides were found to function appropriately only at voltages which are too low for practical implementation in rechargeable lithium and lithium-ion battery cells.
Upon further consideration of the economic advantages possibly attainable in transition metal compounds, interest shifted to examination of the more active fluoride compounds. Investigations into such use of these fluorides confirmed, however, that, while the open structures of the transition metal fluorides support the good ionic conductivity essential, in part, for useful electrode performance, the large band gap induced by the highly ionic character of the metal:halogen bond results in poor electronic conductivity. Without this latter essential conductive property to complement proven ionic conductivity, the transition metal fluorides were considered virtually useless as lithium battery electrode materials.
Despite the inconsequential performance of the transition metal fluorides in typical rechargeable cell fabrications, the theoretical promise of output voltages in the range of 3 V, due to the high ionicity of the compound bonds, prompted some further investigations into metal halides for use in electrode compositions. Increasing the electrical conductivity of iron trifluoride (FeF3) was attempted by incorporating it in an electrode composition comprising the admixture of about 25 parts acetylene black to 70 parts of FeF3. Arai et al., 68 J. POWER SOURCES 716-719 (1997). The performance of such a cell, despite the impractically low charge/discharge rate which extended over a 60 hour cycle period, was marginal at a discharge capacity over 4.5 to 2.0 V of only about 80 mAh/g vis-à-vis a theoretical (le transfer) capacity of 237 mAh/g. Subsequent independent fabrication and testing of similar battery cells at more realistic 4 hour cycle rates would yield no more than about 50 mAh/g.
What is needed are electrical energy-storage and delivery systems that provide high specific capacity over wide cycling-speed ranges, voltages, and operating temperatures; are environmentally friendly; and also are readily available at practical processing and fabrication costs.